The present invention relates to rolls which can be used in calenders or analogous machines to apply pressure to running webs of paper, textile material or the like. More particularly, the invention relates to improvements in flexure-resistant rolls of the type wherein a hollow cylindrical shell spacedly surrounds a stationary carrier and the shell accommodates a plurality of bearing elements and pressure generating devices which react against the carrier and urge the bearing elements against the internal surface of the shell in order to counteract the tendency of the shell to flex in response to external stresses. Such stresses are applied by the complementary roll or rolls of a machine wherein the flexure-resistant roll is installed, by the material which is treated during travel through the nip of the rolls, by gravity and/or a combination of such factors. As a rule, the pressure generating devices comprise hydraulic cylinder and piston units wherein the piston reciprocates in the cylinder and a seal prevents uncontrolled escape of hydraulic fluid from the cylinder chamber. It is also known to provide such rolls with swivelling joints which are disposed between the bearing elements and the carrier in order to allow the bearing elements to move with reference to the carrier in several directions, e.g., radially of the shell as well as about an axis which is parallel to or normal to the axis of the shell. Such swivelling joints normally comprise a substantially spherical head which is rigid with the piston of a pressure generating device and a socket which reciprocably receives the head and is machined into or constitutes the cylinder of the respective pressure generating device in such a way that its axis coincides with the axis of the cylinder. Reference may be had, for example, to FIG. 8 of the aforementioned commonly owned copending application Ser. No. 196,123 of Pav et al. (now U.S. Pat. No. 4,328,744 granted May 11, 1982) which shows a spherical head on the carrier and a socket in the bearing element.
Flexure-resistant rolls which embody the aforediscussed features have found widespread acceptance in many industries, especially in plants which employ calenders, smoothing machines and the like for the treatment of webs which consist of paper, textile or synthetic plastic material. The purpose of the pressure generating devices in combination with swivelling joints and bearing elements is to resist deformation of the shell, to compensate for such deformation, to cause the shell to bear against a running web with a predetermined force and/or to allow for radial or sidewise shifting of the shell.
Swiss Pat. No. 560,334 discloses a flexure-resistant roll wherein the bearing elements are rigidly connected with the pistons of the associated pressure generating devices and the cylinders of such devices are rigidly connected with the stationary carrier. That portion of each piston which extends into the chamber of the respective cylinder has a spherical head and the maximum-diameter portion of the head carries a seal which engages the internal surface of the respective cylinder. Such mounting enables the bearing elements to move radially of the shell as well as to perform pivotal movements about one or more axes including the centers of the respective spherical heads. This, in turn, ensures that the exposed surfaces of the bearing elements can remain in contact with or do not move away from adjacent portions of the internal surface of the shell.
When a roll of the above outlined character is in actual use, its shell is often subjected to the action of transverse forces which develop as a result of frictional engagement between the external surface of the shell and the external surface of an adjacent roll and/or as a result of frictional engagement with a web which is caused to advance through the nip between the shell and a neighboring roll. Furthermore, and if the plane in which the shell receives stresses is not a vertical plane, the shell must also withstand pronounced transverse stresses which are attributable to the weight of the parts of the roll and are transmitted to the carrier by way of the bearing elements and the corresponding swivelling joints. The immediate result is that the seals between the cylinders and pistons of the pressure generating devices are subjected to extensive wear in addition to that wear which is attributable to tilting movements of the bearing elements. Therefore, the useful life of such seals is very short or the pressure generating devices allow the escape of excessive quantities of hydraulic fluid.